You're stunned right now. Get angry.
Do not take what he says about how this is going to go. Do not agree to or sign anything.
Decide what you want. If it is for you to stay in the family home with the children, and him meeting his child maintenance responsibility - FIGHT for that.
You have RIGHTS. Get an appointment with a specialist lawyer who will tell you your rights.
And he has responsibilities. He does not get to just dump the children, or to deprive you and them of your home.
Keep detailed notes of everything he says and does. Someone's warned you he may be recording conversations - you'd be wise to do that yourself.
Tell trusted family and friends. You'll need support, care, a shoulder, etc.
Make sure they know what is actually happening, that he is turning his back on his own children, because he will be manufacturing his own narrative, rewriting history. And he may turn nasty.
If his parents are around let them know. They will not want to lose their grandchildren because their son wants to duck out.